The invention relates to a hydrostatic variable displacement pump of swash plate construction in which the servosystem is integrated in a servosystem housing designed as a cover; the cover closes off a housing opening of the variable displacement pump, a servoarm projecting through said opening for connection to the servosystem.
In the known hydrostatic variable displacement pumps of swash plate construction which operate with a closed circuit, the displacement pistons are guided in cylinders of a cylinder block and rotate about the shaft of the variable displacement pump. During the rotation, the displacement pistons are supported on the swash plate by means of sliding blocks, each displacement piston executing a complete stroke with each 360xc2x0 rotation. The swash plate has a planar running surface on which the sliding blocks slide, the displacement pistons being connected in an articulated manner to the sliding blocks.
The swash plate is usually referred to as a rocker device or adjustable-angle plate, to be precise depending on whether they are mounted in cylinder shells on rollers or can be pivoted about bearing journals. The swash plate is pivoted by adjustment of the servosystem such that the angle position of its running surface is changed in relation to the stroke direction of the displacement pistons. With the change in the angle position of the running surface of the swash plate, the stroke of the displacement pistons, and thus the volume stream produced by the pump, is changed. The force necessary for changing the angle position of the adjustable-angle plate is usually produced hydraulically by virtue of pressure being produced on one, two or possibly even more servopistons which act on the swash plate.
The axis of action of the servopistons is located outside the axis of rotation of the rocker device or adjustable-angle plate, with the result that a lever arm is thus produced. The servopiston or servopistons is/are connected directly or indirectly to the swash plate such that the force displacing the servopistons produces a pivoting torque of the swash plate via the lever arm.
Hydrostatic variable displacement pumps also require spring forces which guide the pivoting angle of the pump back to 0xc2x0, i.e. into the neutral position thereof, if the servo-adjustment means of the pump is not activated.
In pivot-through pumps, the swash plate can be pivot in opposite directions from the center position determined by the spring forces, with the result that two delivery directions are produced. For both delivery directions, there is in each case a volume stream which begins from zero and increases to a specific maximum value. As a result, the center position is also referred to as the zero position. It is often the case that the springs which determine the zero position are installed such that they act as compression springs both in the case of positive pivoting angles and in the case of negative pivoting angles.
Variable displacement pumps in which the springs are accommodated outside the servopiston and servocylinder and are connected to the actual servopiston of the servosystem via corresponding lever systems are known. This means that the springs, rather than acting directly on the moveable servopistons, act on the servopistons via force-deflecting means. (See FIG. 5).
In most of the known variable displacement pumps of the type described above, the springs are installed directly in the servocylinder pressure chamber formed by the servopiston and servocylinder. The springs are thus located in the chamber in which the servopressure for adjusting the angle position of the swash plate also acts. In this case, the springs do indeed act directly on the servopiston, i.e. the spring force is transmitted directly from the spring to the servopiston. However, the dimensioning of the springs is limited by the size of the servocylinder pressure chamber. This means that the spring force cannot be adapted to different force conditions and sizes of the servosystem independently of the size of the servocylinder pressure chamber.
In order that the springs always operate as compression springs regardless of the pivoting direction of the servosystem, they are prestressed between two spring plates. In such devices, a rod is located between the spring plates with a low as possible amount of axial displacement play. The two spring plates can be moved toward one another both in the servopiston and on the rod, but they cannot move apart from one another beyond the distance between the two spring plates. The rod is connected to the housing of the variable displacement pump such that it cannot be displaced axially. In this case, the rod has to be adjusted such that the springs, prestressed to the length between the two spring plates, position the swash plate such that the stroke of the displacement pistons becomes zero (see FIG. 4).
A disadvantage of this configuration is that the spring-force requirement, which determines the geometrical dimensions of the springs, also has an influence on the amount of space required in the servopiston and/or in the servocylinder. This produces an undesirable relationship between the necessary spring force and the necessary servopiston diameter with a corresponding servopiston stroke, which relationship restricts the flexibility of design relatively pronounced extent and cannot be broken up as desired by means of construction. This means that a large spring force also always requires corresponding large servopiston and servocylinder, and a large servocylinder pressure chamber. High spring forces for smaller servopistons and servocylinders are barely possible with such known systems. A further disadvantage of this configuration is that the overall space necessary for the spring produces a dead volume in the servocylinder pressure chamber. In particular in the case of large springs, the dead volume is often greater than the displacement volume of a servopiston stroke. As a result, the servocylinder pressure chamber is not emptied to the full extent during the stroke of the servopiston. If, for example, air is located in this chamber, then air extraction must additionally be ensured by corresponding design measures.
Also known are variable displacement pumps (see FIG. 3), in which the servopiston is arranged in the interior of the tank chamber of the variable displacement pump. There is provided a pivot-back piston against which the servocylinder operates and wherein the spring is arranged on the outer circumference of the piston (see FIG. 3). Double-acting servopistons with inner springs are also known, wherein the servopiston virtually always is arranged at right angles to the pump axis. The application of force for the lever arm in relation to the swash plates should be located as far as possible, as should the center line of the springs, on the center longitudinal axis of the servopiston, in order that the hydraulically mechanical forces on the servopiston do not try to press the servopiston onto the wall of the servocylinder and thus increase friction and wear. This is only expedient in practice, however, when all the springs are located on one side of this application of force (see, in particular, FIG. 4).
In another prior art system the hydrostatic variable displacement pump has a cylinder block in which displacement pistons are guided and circulate with the cylinder block. The displacement pistons are supported on a swash plate, which can have its angle position pivoted in relation to the stroke direction of the displacement pistons, with the result that during the 360xc2x0 rotation of the cylinder block, in which the displacement pistons execute a complete stroke, the stroke of said pistons can be adjusted.
The above servosystem has a piston device with at least two servopiston surfaces subjected to the action of pressure, the servopiston surfaces either being assigned to a single servopiston or each belonging to a separate servopiston. The spring device is arranged outside the servocylinder pressure chamber, around the servopiston, and two such devices are supported on the servosystem housing. This means that the servopiston is arranged wholly or partially in the interior of the compression springs, with the result that the size of the servocylinder pressure chamber is independent of the size of the spring and thus of the adjustable or selectable spring force. The forces of the compression springs are selected here such that they can guide back the pivoting angle of the variable displacement pump to the angle position 0xc2x0 if the servo-adjustment means, i.e. the servosystem of the variable displacement pump, is not activated.
The spring device is installed outside the servopiston, with prestressing, between two spring plates, with the result that a compressive force is exerted on the adjustment piston in each position of the same. The servopiston itself may also be of split configuration here. For reliable abutment of the springs, the spring plate is arranged around the lateral surface of the piston. For reliable abutment of the spring plates, the servopiston is preferably of narrowed design. If the servopiston is not of narrow design, then the abutment for the spring plates may be established by securing rings or comparable elements in the axial direction of the servopiston, it being possible for the securing rings or the comparable elements to be inserted in grooves on the outer circumference of the servopiston, with the result that it is possible to select or even adjust the prestressing of the springs in accordance with the distance between the two spring plates.
The principal object of the invention is thus to provide a variable displacement pump having a servosystem in which it is possible to provide large spring forces, with the spring forces flowing directly to the servopiston, even if small servopistons.
A further object of the invention is to provide for the springs to be installed such that it is also possible for the servochambers to be sufficiently small to be emptied to the full extent, during a stroke of the servopiston.
According to the invention, the closed-circuit hydrostatic variable displacement pump has a servosystem in which either a double-acting servopiston or two servopistons acting directly or indirectly against one another are provided in the servo-adjustment means, the servopistons, being forced into the zero position by spring devices which always act as compression springs during each servopiston stroke, in any desired direction.
Two spring plates act on the servopiston with the corresponding prestressing, are, at the same time, supported in the servosystem housing of the variable displacement pump such that there is no axial displacement play (or at most a very small amount) in the xe2x80x9cservopiston with compression springxe2x80x9d structural unit without the prestressing of the spring changing.
The servopiston is preferably designed as a single-part piston which is double-acting, with the result that two piston surfaces subjected to the action of pressure are provided. Each piston surface subjected to the action of pressure is preferably assigned a spring device. The maximum displaceability of the spring plates for the respective abutment of the spring device is ensured by corresponding shoulders in the interior of the servosystem housing, it being the case that, on account of the prestressing, the springs always act counter to the deflection of the angle position of the swash plate and try to force the latter back into its zero position.
The servosystem is fitted on the housing of the variable displacement pump. It preferably closes off the tank chamber of the variable displacement pump in the outward direction. It thus closes off the opening of the housing of the variable displacement pump through which a servoarm projects, the servosystem acting on said servoarm for adjusting the angle position of the swash plate. The housing of the variable displacement pump also has a large installation opening through which the installation unit comprising the swash plate and its corresponding bearings can be installed in the housing of the variable displacement pump. Corresponding bearing seats for accommodating the bearings are located in the housing of the variable displacement pump, with the result that alignment errors between the bearings mounting the swash plate can be avoided to the greatest possible extent. The servosystem has a servopiston for adjusting the angle of the swash plate, the servopiston, which operates in a servocylinder, forming a servocylinder pressure chamber together with the servocylinder and being displaceable, with hydraulic actuation, counter to the force of a spring device.
According to a development of the invention, the servoarm of the swash plate is arranged in the servosystem such that it is connected in an articulated manner to the servopiston along the line of action of the force exerted by the spring device. This means that the servoarm projects through the opening in the housing of the variable displacement pump, it being possible for the opening to be closed off by the servosystem, integrated in a cover, with the spring-device configuration. The servosystem housing, however, is preferably designed such that, in the state in which it is positioned on the housing of the variable displacement pump and fastened, it is open in the direction of the tank chamber of the variable displacement pump, with the result that the spring chamber is in fluid connection with the tank chamber of the variable displacement pump, but is sealed in the direction of the servocylinder pressure chamber by the actual servosystem piston guided in the pressure cylinder.
A spring plate which has an angled portion is preferably arranged between the servosystem housing and that side of the spring device which is directed toward the surface of the servopiston which is subjected to the action of pressure. The bottom collar of the pressure cylinder, which is screwed into the servocylinder housing, forms, for the angled spring plate, an abutment shoulder of the servosystem housing with respect to the axial displaceability of said spring plate.
The spring device is preferably at least one helical spring. However, it is also possible for two, three or more compression springs to be arranged around the outer circumference of the servopiston, with the result that a single helical spring can be replaced by at least two or more compressions springs.
In order for it to be possible to ensure a play-free spring arrangement, the distance between the shoulders in the servosystem housing is equal to the distance between the spring-plate abutments. A further advantage of the apparatus according to the invention is that the fact that the servosystem housing is open in the direction of the tank chamber of the variable displacement pump also results in a high level of flexibility as far as the dimensions of the spring arranged around the servopiston are concerned.
It is also an essential advantage of the apparatus that on account of the spring-device arrangement separated by the servocylinder pressure chamber, free selection is possible.